This invention relates to contactless data carriers such as smart cards. Contactless smart cards are portable data carriers which contain an integrated circuit and which are capable of establishing a contactless or wireless communication dialogue with a reader.
In contactless smart card applications, a problem to be solved is the likely collision of information sent back to a single common reader by multiple smart cards which the magnetic field generated by the reader for powering the smart cards and for providing clock information thereto. Smart cards which use this power are likely to begin to xe2x80x9ctalk backxe2x80x9d to the reader and, since the smart cards may not be aware of each others, existence while sending information back to the reader, this could result in data collision and hence the reader could receive erroneous messages from the smartcards.
Such a condition is likely to occur, for example, when a person""s wallet or suitcase is waved over the reader. If the wallet contains more than one contactless smart card, then all those smart cards are likely to power up and begin to dialogue with reader in order to complete the desired transaction.
It is an object of the present invention to provide a method of communicating with a plurality of contactless data carriers wherein the above disadvantages nay be overcome or at least alleviated.
In accordance with a first aspect of the invention there is provided a method of communicating with a plurality of contactless data carriers as claimed in claim 1.
In accordance with a second aspect of the invention there is provided a contactless data carrier for use in communicating with a base station as claimed in claim 2.